- 1. The I/O pin is set to output high. A Timer is read to mark the start time.
- 2. The I/O pin is set to input mode with negative edge interrupt enabled. The resistor then discharges the capacitive pad. The TI MSP430 microcontroller goes into low power mode for reduced power consumption, however, it's still able to monitor the interrupt enabled input I/O pins.
- 3. When the voltage of the sensor crosses the low level voltage threshold, an interrupt is generated.
- 4. The interrupt service routine (ISR) reads the Timer and calculates the time to discharge to the low voltage threshold level. This is referred to as a “count” value. The MSP430 exits low power mode and continues operation.

In one embodiment, upon host boot-up, a measurement of each of the analog keys may be performed sequentially (“scanning process”) as labeled instep512. This scanning process may be performed multiple times (e.g., 100 times) in order to allow the master clock and PCB conditions to stabilize.

Though not required, for some applications it may be advisable to provide additional filtering of the “count” measurement to remove any residual noise and further increase sensitivity of the capacitive pads as capacitive pad measurements are often noisy due to a number of factors such as temperature, humidity, voltage drift, component tolerances and 50/60 Hz mains. Instep514, a base capacitance is established and tracked, as the base capacitance of the capacitive pad can change due to environmental conditions such as temperature, humidity, voltage drift and/or component tolerances. Note that this is a slow type of change as changes occur in minutes, not microseconds. A baseline capacitance is established as the capacitance of each pad during the open state (when no finger is pressing on the key). As any of the above mentioned environmental factors changes, the base capacitance for each pad is updated and stored. If a decrease in capacitance is detected, the software must adjust the base capacitance rapidly since this is not a function of the sensor excitation. We can do this be re-averaging with the current count result. If an increase in capacitance is detected, the base capacitance may be adjusted very slowly as this may be due to a finger hovering over a key, and not because of an environmental drift effect. For example, the base capacitance may be adjusted by 1 with each measurement, but only if no keys are pressed. Additionally, an optional low pass filter (e.g., implemented in firmware/software or otherwise), may also help to eliminate the presence of any 50/60 Hz main-power noise that may coupled onto the capacitive pads. For example, in one exemplary embodiment, the low pass noise filter may be implemented as a software based IIR (infinite impulse response) filter, or essentially as a DC tracking filter.

Finally, instep516, sensing for user pressure on each ofanalog keys104 is started. The endless loop of this sensing process is described further below in relation toFIG. 6.

FIG. 6 illustratesmethodology600 for sensing, in real time, pressure applied toanalog keys104, and producing a toggled (alternating open/short)digital signal133 representative thereof.FIG. 6 starts instep602 by taking an averaged capacitance measurement for each pressure sensitive key104 from signals131 based on timer counts in a round robin manner as previously described. This may be done by actually taking multiple (e.g., two) measurements at a given pad. If these multiple measurements are conducted in quick succession, the average behaves like a differential measurement, thus helping filter out common-mode noise. Next, instep604, the baseline count may be evaluated and updated per pad, as per the rules described above, and run through the low pass filter to generate a filtered count output (also called the adjusted capacitance measurement). Instep606, the present adjusted capacitance measurement value for each key104 is then stored, e.g., in memory of pressure-sensingdigital output circuitry190. Next, instep608, all the stored present adjusted capacitance values are examined to determine key(s)104 having a present value of adjusted capacitance that exceeds a pre-determined adjusted capacitance threshold (this is the currently pressed key(s)104). Then instep610 the present identity (e.g., key number) of each of the key(s)104 meeting the threshold adjusted capacitance value is stored along with corresponding present capacitance value(s). Alternatively, the key104 being currently pressed may be identified as the key104 having the largest adjusted capacitance measurement of allkeys104 in the current round robin cycle, i.e., rather than using the adjusted capacitance threshold value to determine pressed keys.

Example code for executing steps602-610 is as follows:


/* TAKE MEASUREMENT ON 4 CAPACITIVE PADS */
int scan_keys(void)
{

	int i;
	int margin;
	for (i = 0; i < NUM_LINES; i++)
	{

/* take pad measurement and establish */

margin = measure_key_capacitance(i) − key_line[i].base_capacitance;

/* convert measurement to filtered value using a single pole IIR low pass

	filter */
	key_line[i].filtered += (margin − (key_line[i].filtered >> 4));

/* KEY_LINE[I].FILTERED IS OUR “COUNT” WE WANT TO USE */

	}
	return 0;

}

/* FIGURE OUT WHICH KEY LINE IS PRESSED ON (IF ANY) */

int find_finger_position(void)

{

	int i;
	int min;
	int max;
	int max_pos;
	/* Find the minimum and maximum responses for all the 4 key lines */
	min = 32767;
	max = −32768;
	max_pos = −1;
	for (i = 0; i < NUM_LINES; i++)
	{

if (key_line[i].filtered < min)

min = key_line[i].filtered;

	if (key_line[i].filtered > max)
	{

	max = key_line[i].filtered;
	max_pos = i;

}

	}
	/* If the maximum response isn't that big, there is no finger present. */
	if (max < 200)
	{

	P1OUT &= 0xCF; /* no key pressed = “00” (P1.5, P1.4) */
	P1DIR \|= 0x30;
	return 0;

	}
	/P1.5, P1.4 are the 2 bit output pins/
	/* TRUTH TABLE BELOW FOR KEY PRESS */
	/* P1.5 P1.4

	0 0 NO KEY IS PRESSED
	1 0 P1.0 IS PRESSED
	1 1 P1.2 IS PRESSED
	0 1 P1.3 IS PRESSED

	*/
	if (max_pos == 0)
	{

	P1OUT \|= 0x20; /* P1.0 IS PRESSED = “10” */
	P1OUT &= 0xEF;
	P1DIR \|= 0x30;
	return max;

	}
	if (max_pos == 2)
	{

	P1OUT \|= 0x30; /* P1.2 IS PRESSED = “11” */
	P1DIR \|= 0x30;
	return max;

	}
	if (max_pos == 3)
	{

	P1OUT &= 0xDF; /* P1.3 IS PRESSED = “01” */
	P1OUT \|= 0x10;
	P1DIR \|= 0x30;
	return max;

	}
	if (max_pos == 1)
	{

	P1OUT &= 0xCF; /* otherwise = “00” */
	P1DIR \|= 0x30;
	return 0;

	}
	else
	{

	P1OUT &= 0xCF; /* otherwise = “00” */
	P1DIR \|= 0x30;
	return 0;

}

int pressed_key_pressure = 0;

void main(void)

{

.

/* INITIALIZE ALL PADS TO BE SCANNED TO GROUND */

for (i = 0; i < NUM_LINES; i++)

init_key(&key_line[i], &key_line_config[i]);

	TACTL = TASSEL_2 \| MC_2; // \| ID_3;
	/* Scan thekeys 100 times, allowing plenty of time for the MCLK and board
	conditions to stablise */
	for (i = 0; i < 100; i++)

scan_keys( );

	/* Establish base capacitance and filtered “count” per active pad */
	for (i = 0; i < NUM_LINES; i++)
	{

	key_line[i].base_capacitance = key_line[i].filtered >> 4;
	key_line[i].filtered = 0;

	}
	for (;;)
	{

	scan_keys( );
	if ((pressed_key_pressure = find_finger_position( )) > 0)
	{

	/* There is a finger on the pad */
	send_to_host(pressed_key_pressure);

	}
	else
	{

	/* There is no finger on the pad */
	send_to_host(0);

}

Still referring toFIG. 6, the degree of applied pressure to a presently-pressed key is next determined, e.g., in a binning operation by comparing the filtered count value to a pre-determined scale of counts per resolution instep612 based on the stored adjusted capacitance value (or filtered count value) ofstep610. Then, instep614, a lowactive output133 of pressure-sensingdigital output circuitry190 is toggled that corresponds to the identity of the presently-pressed key (e.g., output P2.1 is toggled for corresponding input P1.1). This toggled signal mimics or emulates the action of a user repeatedly pressing a conventionaldigital key106 for a number of times that is proportional or otherwise relative to the strength of the desired input (i.e. greater pressure onanalog key104 corresponds to more rapid user repeat rate on digital key106).

For example, in one exemplary embodiment, four levels of different toggle output resolution (i.e., alternating toggle rate frequency) may be pre-defined for measured timer counts of a pressedkey104. As an example, a maximum toggle rate may be defined to correspond to a maximum timer count level of 200 with four decreasingly lower toggle rate levels defined for timer counts of 180, 160, 140 and 120, and anything below 120 being disregarded as noise, e.g., toggle rate frequency of 20 times/second for timer count range of 180-200, toggle rate frequency of 16 times/second for timer count range of 160-180, toggle rate frequency of 10 times/second for timer count range of 140-160, toggle rate frequency of 8 times/second for timer count range of 120-140, and no toggling for timer count range of less than 120. It will be understood that this particular number of timer count levels and corresponding timer count values is exemplary only and that greater or fewer numbers of timer count levels and/or different timer count values may be employed in other embodiments. Following is example code for the four timer count level embodiment described above:


if (count > 180)

for (num_toggles = 20; num_toggles >0 ; num_toggles−−)

{

	P2OUT {circumflex over ( )}= 0x02; /*Toggle P2.1 low 10 times in
	1 sec */
	/* loop provides delay of 0.05 sec/.000017922 sec = 2789
	loops */
	for (loop=2789; loop>0; loop − −);
	}

else if ((count > 160) && (count <= 180))

for (num_toggles=16; num_toggles>0; num_toggles − −)

{

	P2OUT {circumflex over ( )}= 0x02; /*Toggle P2.1 low 8 times in
	1 sec */
	/* loop provides delay of 0.0625 sec/.000017922 sec =
	3487 loops */
	for (loop=3487; loop>0; loop − −);
	}

else if ((count > 140) && (count <= 160))

for (num_toggles=10; num_toggles>0; num_toggles − −)

{

	P2OUT {circumflex over ( )}= 0x02; /*Toggle P2.1 low 5 times in
	1 sec */
	/* loop provides delay of 0.1 sec/.000017922 sec =
	5579 loops */
	for (loop=5579; loop>0; loop − −);
	}

else if ((count > 120) && (count <= 140))

for (num_toggles=4; num_toggles>0; num_toggles − −)

{

	P2OUT {circumflex over ( )}= 0x02; /*Toggle P2.1 low 2 times
	in 1 sec */
	/*loop provides delay of 0.25 sec/.000017922 sec =
	13949 loops */
	for (loop=13949; loop>0; loop − −);
	}

else

{

/ * DO NOTHING. KEY IS NOT PRESSED ON */

	}

It will be understood thatstep610 described above is optional and only may be employed when the number ofanalog keys104 exceeds the number ofoutput lines133. Alternatively,methodology600 may: pause the round robin measurements whenever a pressed key104 is identified as exceeding the threshold adjusted capacitance value, determine the degree of applied pressure to pressed key104 based on its capacitance, and toggle the output signal of the pressedkey104. The count value of the identified pressed key104 may be monitored and reevaluated for as long as it remains pressed by reevaluating the pressed key's timer count by rerunning it through the binning operation to see if the pressure is changed, and outputting an updated digitalbit stream signal133 based thereon. Once it is determined that the identifiedkey104 is no longer being pressed, then the round robin procedure may resume to thenext key104 and inquire of its count value for this round robin cycle. Either way, the round robin cycle continues for as long as thekeyboard system100 is powered up. After shut down, the pressure-sensingdigital output circuitry190 may be reset on next power up, all keys reinitialized (e.g., perFIG. 5), and the round robin key capacitance measurement routine initialized again.

It will be understood that one or more of the tasks, functions, or methodologies described herein may be implemented, for example, as firmware or other computer program of instructions embodied in a tangible computer readable medium that is executed by a CPU, microcontroller, or other suitable processing device.

Further modifications and alternative embodiments of the techniques described herein will be apparent to those skilled in the art in view of this description. It will be recognized, therefore, that the techniques described herein are not limited by these example arrangements. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the manner of carrying out the techniques described herein. It is to be understood that the forms of the techniques described herein shown and described are to be taken as the presently preferred embodiments. Various changes may be made in the implementations and architectures. For example, equivalent elements may be substituted for those illustrated and described herein and certain features of the techniques described herein may be utilized independently of the use of other features, all as would be apparent to one skilled in the art after having the benefit of this description of the techniques.